For effectively impregnating a fibrous base material, especially a fibrous sheet-like base material, with varnish liquid, it is essential that the varnish liquid is internally absorbed into and uniformly distributed throughout the fibrous base material. Air bubbles trapped in the base material interfere with such impregnation and, therefore, must be eliminated to the extent possible.
In common varnish impregnating processes, a base material is preliminarily impregnated with an ordinary primer varnish to remove air bubbles and then fed via a timing roll into a varnish liquid for impregnation.
Those known varnish impregnating processes, however, have drawbacks, such that: (A) a uniform and sufficient impregnation of the base material with the varnish is not easy and the processes take long times; and (B) the air bubbles trapped in the base material are not sufficiently eliminated. Those drawbacks are more evident when a higher viscosity varnish is employed.
Another process has recently been introduced to the art, as shown in FIG. 3, in which a base material 11 is guided by guide rolls 17 to run through a preliminary impregnating tank 13' filled with a solvent or a varnish diluted with a solvent 13'a, referred to in the art as "preliminary impregnating liquid". When a diluted varnish is used, it contains a large amount of solvent, as opposed to an ordinary varnish liquid, which has been, in fact, used in the past for preliminary impregnation. A main varnish impregnating tank 13 is filled with an ordinary varnish liquid 13a.
In this process, air trapped in the base material 11 is displaced by the preliminary impregnating liquid 13'a and eliminated during the passing of the base material 11 through the preliminary impregnating liquid 13'a. Thus, by subsequently feeding the base material 11 into the ordinary varnish liquid 13a and, simultaneously, evaporating preliminary impregnating liquid 13'a absorbed in the base material 11, impregnation with the ordinary varnish 13a will be relatively efficient and air bubbles will be reasonably eliminated.
However, in the process shown in FIG. 3, the base material 11, during transition from the preliminary impregnating liquid 13'a to the ordinary varnish liquid 13a, is exposed to the air and, also, pressed by guide roll 17', arranged between the two tanks 13 and 13'. As a result, the removal of air from the base material 11 is obstructed and thus, the impregnation with ordinary varnish and providing an air-free base material is hindered.
More particularly, while the base material 11 runs on the guide roll 17', a contact pressure is exerted by the roll onto each fiber bundle of the base material 11. This pressure opens the fiber bundles and causes the preliminary impregnating liquid 13'a, retained in the fiber bundles and fiber strands by capillary action, to be displaced. When the opened fiber bundles are again formed into closed fiber bundles, as the fiber bundles are freed from the pressure exerted by the guide roll 17' (after passing that guide roll), both the displacement of the preliminary impregnating liquid and the restoration of the closed fiber bundles are executed in the air. This allows re-permeation of air through the base material to inevitably occur during the transition from the preliminary impregnating liquid 13'a to the ordinary varnish liquid 13a.
The present invention is directed, in view of the foregoing problems, to a varnish impregnating method and apparatus having substantial improvements both in the impregnation of the base material with ordinary varnish and in the air-free condition of the varnish impregnated base material.